Control system for hybrid construction machine

ABSTRACT

A controller is connected to a pressure sensor. The controller controls a regulator of a sub-pump in accordance with a pressure signal from the pressure sensor, detects an output of a main pump, and controls an output of an electric motor according to the output of the main pump based on a table stored beforehand.

This is a Continuation of U.S. application Ser. No. 13/512,863, filed onMay 30, 2012, which was a National Stage application ofPCT/JP2011/052494, filed Feb. 7, 2011, the subject matters of which areincorporated herein by reference.

TECHNICAL FIELD

This invention relates to a control system for hybrid constructionmachine including a sub-pump which is rotated by a drive force of anelectric motor.

BACKGROUND ART

JP2009-235717A discloses a control system for hybrid constructionmachine.

In this conventional control system, oil discharged from avariable-capacity sub-pump is joined at a discharge side of avariable-capacity main pump and the sub-pump is driven by an electricmotor. A tilting angle of the main pump is controlled by the action of apilot pressure generated according to an operated amount of a controlvalve.

An assist force of the sub-pump for the main pump is set beforehand soas to be most efficient in response to the pilot pressure.

SUMMARY OF INVENTION

In this conventional control system, the assist force of the sub-pumpcorresponds to the pilot pressure of the main pump. However, since beingset beforehand, this assist force does not change even if a workingstate such as light work and heavy work changes. Therefore, an assistpump gives an output more than necessary even at the time of light work,whereby battery consumption increases.

Since the electric motor is driven by power of a battery and the life ofthe battery is proportional to a cumulative amount of consumed power, ifpower is consumed more than necessary at the time of light work, thelife of the battery is shortened by that much.

This invention aims to control an output of an electric motor as a drivesource for a sub-pump according to a working state such as light workand heavy work to reduce battery consumption and extend the life of abattery in a control system for hybrid construction machine.

One aspect of the present invention is directed to a control system forhybrid construction machine, including a variable-capacity main pump; acircuit system which is connected to the main pump and includes aplurality of control valves; a regulator which controls a tilting angleof the main pump; a pilot flow path which is provided in the circuitsystem and introduces a pilot pressure generated when any one of theplurality of control valves is switched to the regulator; an electricmotor; a variable-capacity sub-pump which is connected to a dischargeside of the main pump and driven by an output of the electric motor; aregulator which is provided in the sub-pump and controls a tilting angleof the sub-pump; a pressure sensor which is provided in the pilot flowpath and detects the pilot pressure; and a controller which is connectedto the pressure sensor, controls the regulator of the sub-pump inaccordance with a pressure signal from the pressure sensor, detects anoutput of the main pump and controls the output of the electric motoraccording to the output of the main pump based on a table storedbeforehand.

According to the above aspect, an assist force of the electric motor canbe controlled, for example, according to a working state such light workor heavy work. Thus, an assist force more than necessary is notexercised at the time of light work and battery consumption is reducedby that much.

Since the output of the electric motor can be relatively reduced at thetime of light work, it is also possible to extend the life of a battery.

An embodiment of the present invention and advantages thereof aredescribed in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a hydraulic circuit diagram showing an embodiment of thepresent invention,

FIG. 2 is a graph showing a relationship between displacement volume ofan assist motor and pressure by return oil of a boom cylinder,

FIG. 3 is a graph showing a relationship between relief flow rate of arelief valve and pressure by the return oil of the boom cylinder, and

FIG. 4 is a flow chart showing control contents of a controller.

EMBODIMENT OF INVENTION

An embodiment shown in FIG. 1 is a control system for a power shovel.The control system includes variable-capacity first and second mainpumps MP1, MP2. A first circuit system is connected to the first mainpump MP1, and a second circuit system is connected to the second mainpump MP2.

To the first circuit system are connected a control valve 1 forcontrolling a rotation motor RM, a control valve 2 for controlling anunillustrated arm cylinder, a control valve 3 for controlling a boomcylinder BC, a control valve 4 for controlling an unillustratedauxiliary attachment and a control valve 5 for controlling anunillustrated left travel motor in this order from an upstream side.

The respective control valves 1 to 5 are connected to the first mainpump MP1 via a neutral flow path 6 and a parallel passage 7.

A pilot pressure generating mechanism 8 is provided downstream of thecontrol valve 5 in the neutral flow path 6. The pilot pressuregenerating mechanism 8 generates a high pilot pressure if a flow ratetherethrough is high while generating a low pilot pressure if the flowrate is low.

The neutral flow path 6 introduces all or part of fluid discharged fromthe first main pump MP1 to a tank T when all the control valves 1 to 5are at or near neutral positions. In this case, a high pilot pressure isgenerated since the flow rate through the pilot pressure generatingmechanism 8 is also high.

If the control valves 1 to 5 are switched to full-stroke states, theneutral flow path 6 is closed and fluid does not flow any longer. Inthis case, the flow rate through the pilot pressure generating mechanism8 is almost zero and the pilot pressure is kept at zero.

However, depending on the operated amounts of the control valves 1 to 5,part of pump-discharged amount is introduced to an actuator and part ofit is introduced to the tank T from the neutral flow path 6. Thus, thepilot pressure generating mechanism 8 generates a pilot pressurecorresponding to the flow rate in the neutral flow path 6. In otherwords, the pilot pressure generating mechanism 8 generates the pilotpressure corresponding to the operated amounts of the control valves 1to 5.

A pilot flow path 9 is connected to the pilot pressure generatingmechanism 8. The pilot flow path 9 is connected to a regulator 10 forcontrolling a tilting angle of the first main pump MP1. The regulator 10controls the discharge amount of the first main pump MP1 in inverseproportion to a pilot pressure. The discharge amount of the first mainpump MP1 is kept maximum when the control valves 1 to 5 are set to thefull stroke states so that the flow in the neutral flow path 6 becomeszero, in other words, when the pilot pressure generated by the pilotpressure generating mechanism 8 becomes zero.

A first pressure sensor 11 is connected to the pilot flow path 9. Apressure signal detected by the first pressure sensor 11 is input to acontroller C.

To the second circuit system are connected a control valve 12 forcontrolling an unillustrated right travel motor, a control valve 13 forcontrolling an unillustrated bucket cylinder, a control valve 14 forcontrolling the boom cylinder BC, and a control valve 15 for controllingthe unillustrated arm cylinder in this order from an upstream side. Asensor 14 a for detecting an operating direction and an operated amountof the control valve 14 is provided in the control valve 14.

The respective control valves 12 to 15 are connected to the second mainpump MP2 via a neutral flow path 16. The control valve 13 and thecontrol valve 14 are connected to the second main pump MP2 via aparallel passage 17.

A pilot pressure generating mechanism 18 is provided downstream of thecontrol valve 15 in the neutral flow path 16. The pilot pressuregenerating mechanism 18 functions in just the same manner as the pilotpressure generating mechanism 8.

A pilot flow path 19 is connected to the pilot pressure generatingmechanism 18. The pilot flow path 19 is connected to a regulator 20 forcontrolling a tilting angle of the second main pump MP2. The regulator20 controls the discharge amount of the second main pump MP2 in inverseproportion to a pilot pressure. Accordingly, the discharge amount of thesecond main pump MP2 is kept maximum when the control valves 12 to 15are set to the full stroke states so that the flow in the neutral flowpath 16 becomes zero, in other words, when the pilot pressure generatedby the pilot pressure generating mechanism 18 becomes zero.

A second pressure sensor 21 is connected to the pilot flow path 19. Apressure signal detected by the second pressure sensor 21 is input tothe controller C.

The first and second main pumps MP1, MP2 are coaxially rotated by adrive force of one engine E. The engine E includes a generator 22. Thegenerator 22 is rotated by excess power of the engine E to generatepower. Power generated by the generator 22 is charged into a battery 24via a battery charger 23.

The battery charger 23 can charge the battery 24 with power also whenbeing connected to a normal household power supply 25. That is, thebattery charger 23 is also connectable to another independent powersupply.

Passages 26, 27 communicating with the rotation motor RM are connectedto an actuator port of the control valve 1 connected to the firstcircuit system. Brake valves 28, 29 are respectively connected to theboth passages 26, 27. When the control valve 1 is kept at the shownneutral position, the actuator port is closed and the rotation motor RMremains stopped.

If the control valve 1 is switched, for example, to a right position inFIG. 1 from the above state, one passage 26 is connected to the firstmain pump MP1 and the other passage 27 communicates with the tank T.Accordingly, the rotation motor RM rotates by having pressure fluidsupplied from the passage 26, and return fluid from the rotation motorRM is returned to the tank T via the passage 27.

If the control valve 1 is, conversely, switched to a left position, thenpump-discharged fluid is supplied to the passage 27, the passage 26communicates with the tank T, and the rotation motor RM rotates in areverse direction.

When the rotation motor RM is driven, the brake valve 28 or 29 functionsas a relief valve. When pressures in the passages 26, 27 increase to setpressures or higher, the brake valves 28, 29 are opened to introducefluid at a high-pressure side to a low-pressure side. If the controlvalve 1 is returned to the neutral position while the rotation motor RMis being rotated, the actuator port of the control valve 1 is closed.Even if the actuator port of the control valve 1 is closed, the rotationmotor RM continues to rotate due to inertial energy thereof. By rotatingdue to inertial energy, the rotation motor RM functions as a pump. Inthis case, a closed circuit is formed by the passages 26, 27, therotation motor RM and the brake valve 28 or 29, and the inertial energyis converted into thermal energy by the brake valve 28 or 29.

If the control valve 14 is switched to a right position in FIG. 1 fromthe neutral position, pressure fluid from the second main pump MP2 issupplied to a piston-side chamber 31 of the boom cylinder BC via apassage 30. Return fluid from a rod-side chamber 32 is returned to thetank T via a passage 33, whereby the boom cylinder BC extends.

On the contrary, if the control valve 14 is switched to a left positionin FIG. 1, pressure fluid from the second main pump MP2 is supplied tothe rod-side chamber 32 of the boom cylinder BC via the passage 33.Return fluid from the piston-side chamber 31 is returned to the tank Tvia the passage 30, whereby the boom cylinder BC contracts. The controlvalve 3 is switched in association with the control valve 14.

A proportional electromagnetic valve 34, the opening of which iscontrolled by the controller C, is provided in the passage 30 connectingthe piston-side chamber 31 of the boom cylinder BC and the control valve14. The proportional electromagnetic valve 34 is kept at a fully openposition in a normal state.

Next, a variable-capacity sub-pump SP for assisting outputs of the firstand second main pumps MP1, MP2 is described.

The sub-pump SP is rotated by a drive force of an electric motor MGdoubling as a generator. A variable-capacity assist motor AM is alsocoaxially rotated by the drive force of the electric motor MG. Aninverter I is connected to the electric motor MG. The controller C isconnected to the inverter I and the rotation speed of the electric motorMG and the like can be controlled by the controller C.

Titling angles of the sub-pump SP and the assist motor AM are controlledby regulators 35, 36. The regulators 35, 36 are controlled by outputsignals of the controller C.

A discharge passage 37 is connected to the sub-pump SP. The dischargepassage 37 is branched off to a first joint passage 38 which joins at adischarge side of the first main pump MP1 and a second joint passage 39which joins at a discharge side of the second main pump MP2. First andsecond proportional electromagnetic throttle valves 40, 41, the openingsof which are controlled by output signals of the controller C, areprovided in the respective first and second joint passages 38, 39.

A connection passage 42 is connected to the assist motor AM. Theconnection passage 42 is connected to the passages 26, 27 connected tothe rotation motor RM via the joint passage 43 and check valves 44, 45.An electromagnetic switching valve 46, the opening and closing of whichare controlled by the controller C, is provided in the joint passage 43.A pressure sensor 47 for detecting a pressure at the time of rotatingthe rotation motor RM or a pressure at the time of braking is providedbetween the electromagnetic switching valve 46 and the check valves 44,45. A pressure signal of the pressure sensor 47 is input to thecontroller C.

In the joint passage 43, a safety valve 48 is provided at a positiondownstream of the electromagnetic switching valve 46 with respect to aflow from the rotation motor RM to the connection passage 42. The safetyvalve 48 prevents runaway of the rotation motor RM by maintaining thepressures in the passages 26, 27 when there is a failure in theconnection passage 42, the joint passage 43 and the like such as afailure of the electromagnetic switching valve 46.

A passage 49 communicating with the connection passage 42 is providedbetween the boom cylinder BC and the proportional electromagnetic valve34. An electromagnetic on-off valve 50 controlled by the controller C isprovided in the passage 49.

The passage 42 includes a passage 51 communicating with the regulator 36for controlling the tilting angle of the assist motor AM. A relief valve52 is provided in the passage 51. A throttle 53 is provided upstream ofthe relief valve 52.

The regulator 36 communicating with the passage 42 reduces adisplacement volume D per rotation as shown in FIG. 2, for example, whena pressure produced by the return oil from the boom cylinder BCintroduced into the passage 42 increases. Accordingly, if a torque Tacting on the assist motor AM is assumed to be T=(D·P)/2π, the regulator36 keeps the torque T at or below an electric motor absorption torque byreducing the displacement volume D when a pressure P increases.

By providing the throttle 53 upstream of the relief valve 52, a pressureoverride of the relief valve 52 is substantially worsened. The pressureoverride is substantially worsened to gradually increase a relief flowrate as shown by solid line in FIG. 3. That is, when the pressureproduced by the return oil from the boom cylinder BC increases in theconnection passage 42, the boom cylinder BC cannot be stopped withoutcausing any sense of incongruity if the relief valve 52 increases therelief flow rate at once as shown by broken line in FIG. 3.

In this embodiment, the assist flow rate of the sub-pump SP is setbeforehand in accordance with pressure signals of the first and secondpressure sensors 11, 21 and the controller C judges how the tiltingangle of the sub-pump SP, that of the assist motor AM, the rotationspeed of the electric motor MG and the like can be most efficientlycontrolled and controls the respective parts.

The controller C of this embodiment detects outputs of the first andsecond main pumps MP1, MP2 and estimates whether the operation is beingperformed in a light working state or a heavy working state from thestates of outputs.

That is, the controller C estimates the outputs of the first and secondmain pumps MP1, MP2 from the discharge pressures and discharge flowrates thereof. The discharge amounts of the first and second main pumpsMP1, MP2 may be directly measured by unillustrated flow rate detectors,but may be inferred from the displacement volumes per rotation of thefirst and second main pumps MP1, MP2 and the rotation speeds thereof atthat time.

Further, a table shown in FIG. 4 is stored beforehand in the controllerC. This table is composed of data on assist correction coefficientscorresponding to the outputs of the first and second main pumps MP1,MP2. The assist correction coefficient is 1 in the case of a heavyworking state while being below 1 in the case of a light working state.

The controller C estimates the outputs of the first and second mainpumps MP1, MP2, specifies an assist correction coefficient correspondingto the values of the outputs, and calculates an assist flow rate powercorrection command value by filtering the assist correction coefficientby a low-pass filter. Then, the controller C controls the output of theelectric motor MG, which drives the sub-pump SP, based on the assistflow rate power correction command value.

Since the outputs of the first and second main pumps MP1, MP2 largelyvary depending on the content of work such as excavation work, acorrection command is issued after suppressing output variations of thefirst and second main pumps MP1, MP2 by the low-pass filter and acontrol is executed while a sudden change of the electric motor MG issuppressed.

If the control valves 1 to 5 of the first circuit system are kept at theneutral positions, all the fluid discharged from the first main pump MP1is introduced to the tank T via the neutral flow path 6 and the pilotpressure generating mechanism 8. If all the fluid discharged from thefirst main pump MP flows through the pilot pressure generating mechanism8, a pilot pressure generated there becomes higher and a relatively highpilot pressure is also introduced to the pilot flow path 9. Theregulator 10 operates by the action of the high pilot pressureintroduced to the pilot flow path 9 to keep the discharge amount of thefirst main pump MP1 minimum. A pressure signal indicating the high pilotpressure at this time is input to the controller C from the firstpressure sensor 11.

Also when the control valves 12 to 15 of the second circuit system arekept at the neutral positions, the pilot pressure generating mechanism18 generates a relatively high pilot pressure as in the case of thefirst circuit system and this high pressure acts on the regulator 20 tokeep the discharge amount of the second main pump MP2 minimum. Apressure signal indicating the high pilot pressure at this time is inputto the controller C from the second pressure sensor 21.

When pressure signals indicating relatively high pressures are inputfrom the first and second pressure sensors 11, 21 to the controller C,the controller C judges that the first and second main pumps MP1, MP2maintain minimum discharge amounts and controls the regulators 35, 36 tozero or minimize the tilting angles of the sub-pump SP and the assistmotor AM.

When receiving signals indicating that the discharge amounts of thefirst and second main pumps MP1, MP2 are minimum, the controller C maystop the rotation of the electric motor MG or may keep it rotating.

In the case of stopping the rotation of the electric motor MG, there isan effect of saving power consumption. In the case of keeping theelectric motor MG rotating, the sub-pump SP and the assist motor AM alsokeep rotating. Thus, there is an effect of reducing a shock at the timeof starting the sub-pump SP and the assist motor AM. In any case,whether to stop the electric motor MG or to keep it rotating isdetermined according to the intended purpose and usage of theconstruction machine.

If any one of the control valves of the first and second circuit systemsis switched in the above state, the flow rate in the neutral flow path 6or 16 decreases according to the operated amount and the pilot pressuregenerated in the pilot pressure generating mechanism 8 or 18 accordinglydecreases. If the pilot pressure decreases, the tilting angle of thefirst or second main pump MP1 or MP2 accordingly increases to increasethe discharge amount.

In the case of increasing the discharge amount of the first or secondmain pump MP1 or MP2, the controller C keeps the electric motor MG in aconstantly rotating state. That is, if the electric motor MG stops whenthe discharge amounts of the first and second main pumps MP1, MP2 areminimum, the controller C detects a decrease in the pilot pressure andrestarts the electric motor MG.

In this case, the controller C calculates a total output of the firstand second main pumps MP1, MP2 and determines whether the total outputis higher or lower than a reference value. It is determined that thefirst and second main pumps MP1, MP2 are driven in a light working stateif the total output is lower than a light work reference value, it isdetermined that they are driven in a heavy working state if the totaloutput is higher than a heavy work reference value, and it is determinedthat they are driven in an intermediate state if work is between lightwork and heavy work.

The controller C calculates an assist flow rate power correction commandvalue corresponding to each working state and controls the output of theelectric motor MG based on the assist flow rate power correction commandvalue.

Accordingly, the controller C drives the electric motor MG bymultiplying a command for the electric motor MG by a correctioncoefficient=1 in heavy work. In light work, the controller C drives theelectric motor MG by multiplying the command by a correction coefficientset beforehand and smaller than that at the time of heavy work. In anintermediate range between light work and heavy work, the controller Cdrives the electric motor MG by multiplying the command by a correctioncoefficient between the small correction coefficient set beforehand andthe correction coefficient=1.

The controller C controls the openings of the first and secondproportional electromagnetic throttle valves 40, 41 in accordance withthe pressure signals of the first and second pressure sensors 11, 21 andproportionally distributes the discharge amount of the sub-pump SP tosupply it to the first and second circuit systems.

Since the controller C can control the tilting angle of the sub-pump SPand the openings of the first and second proportional electromagneticthrottle valves 40, 41 only by the pressure signals of the two first andsecond pressure sensors 11, 21, the number of the pressure sensors canbe reduced.

If the control valve 1 is switched to either left or right, e.g. to theright position in FIG. 1 to drive the rotation motor RM connected to thefirst circuit system, the one passage 26 communicates with the firstmain pump MP1 and the other passage 27 communicates with the tank T torotate the rotation motor RM. In this case, a rotation pressure is keptat a set pressure of the brake valve 28. If the control valve 1 isswitched to left in FIG. 1, the other passage 27 communicates with thefirst main pump MP1 and the one passage 26 communicates with the tank Tto rotate the rotation motor RM. Also in this case, a rotation pressureis kept at a set pressure of the brake valve 29.

If the control valve 1 is switched to the neutral position during therotation of the rotation motor RM, a closed circuit is formed betweenthe passages 26 and 27 and the brake valve 28 or 29 maintains a brakepressure of the closed circuit to convert inertial energy into thermalenergy.

The pressure sensor 47 detects the rotation pressure or the brakepressure and a pressure signal thereof is input to the controller C. Thecontroller C switches the electromagnetic switching valve 46 from aclosed position to an open position in the case of detecting a pressurewhich is within such a range as not to affect the rotation of therotation motor RM or a braking operation and lower than the setpressures of the brake valves 28, 29. If the electromagnetic switchingvalve 46 is switched to the open position, pressure fluid introduced tothe rotation motor RM flows into the joint passage 43 and is supplied tothe assist motor AM via the safety valve 48 and the connection passage42.

The controller C controls the tilting angle of the assist motor AM inaccordance with a pressure signal from the pressure sensor 47. Thiscontrol is as follows.

Unless the pressure in the passage 26 or 27 is kept at a pressurenecessary for the rotating operation or the braking operation, itbecomes impossible to rotate the rotation motor RM or apply braking.

Accordingly, to keep the pressure in the passage 26 or 27 at therotation pressure or the brake pressure, the controller C controls aload of the rotation motor RM while controlling the tilting angle of theassist motor AM. That is, the controller C controls the tilting angle ofthe assist motor AM so that the pressure detected by the pressure sensor47 becomes substantially equal to the rotation pressure of the rotationmotor RM or the brake pressure.

If the assist motor AM obtains a rotational force, this rotational forceacts on the coaxially rotating electric motor MG. The rotational forceof the assist motor AM acts as an assist force for the electric motorMG. Accordingly, power consumption of the electric motor MG can bereduced by the rotational force of the assist motor AM.

The rotational force of the sub-pump SP can also be assisted by therotational force of the assist motor AM. In this case, the assist motorAM and the sub-pump SP exhibit a pressure conversion function together.

That is, the pressure of the fluid flowing into the connection passage42 is invariably lower than a pump-discharged pressure. To maintain ahigh discharge pressure of the sub-pump SP by utilizing this lowpressure, a boosting function is exhibited by the assist motor AM andthe sub-pump SP.

That is, an output of the assist motor AM is determined by a product ofa displacement volume Q1 per rotation and a pressure P1 at that time. Anoutput of the sub-pump SP is determined by a product of a displacementvolume Q2 per rotation and a discharge pressure P2. Since the assistmotor AM and the sub-pump SP are coaxially rotated in this embodiment,Q1×P1=Q2×P2 has to hold. Accordingly, if the displacement volume Q1 ofthe assist motor AM is set, for example, to be three times as much asthe displacement volume Q2 of the sub-pump SP, i.e. Q1=3Q2, the aboveequation is 3Q2×P1=Q2×P2. If the both sides of this equation are dividedby Q2, 3P1=P2 holds.

Accordingly, the sub-pump SP can be maintained at a predetermineddischarge pressure by the output of the assist motor AM by changing thetiling angle of the sub-pump SP and controlling the displacement volumeQ2. In other words, the fluid can be discharged from the sub-pump SPafter boosting the fluid pressure from the rotation motor RM.

The tiling angle of the assist motor AM is so controlled as to keep thepressures in the passages 26, 27 at the rotation pressure or the brakepressure. Thus, the tilting angle of the assist motor AM is inevitablydetermined in the case of utilizing the fluid from the rotation motorRM. To exhibit the pressure conversion function with the tiling angle ofthe assist motor AM determined, the tilting angle of the sub-pump SP iscontrolled.

If a pressure in the connection passage 42, the joint passage 43 or thelike falls below the rotation pressure or the brake pressure due to acertain cause, the controller C closes the electromagnetic switchingvalve 46 in accordance with a pressure signal from the pressure sensor47 so that the rotation motor RM is not affected.

If there is a fluid leak in the connection passage 42, the safety valve48 functions so that the pressures in the passages 26, 27 do notdecrease more than necessary, thereby preventing runaway of the rotationmotor RM.

Next, a case is described where the boom cylinder BC is controlled byswitching the control valve 14 and the control valve 3 of the firstcircuit system in association with the control valve 14.

If the control valve 14 and the control valve 3 associated therewith areswitched to actuate the boom cylinder BC, the sensor 14 a detects anoperating direction and an operated amount of the control valve 14 andan operation signal is input to the controller C.

In accordance with the operation signal of the sensor 14 a, thecontroller C determines whether an operator is trying to raise or lowerthe boom cylinder BC. If a signal for raising the boom cylinder BC isinput to the controller C, the controller C keeps the proportionalelectromagnetic valve 34 in the normal state, in other words, keeps theproportional electromagnetic valve 34 at the fully open position. Inthis case, the controller C keeps the electromagnetic on-off valve 50 atthe shown closed position and controls the rotation speed of theelectric motor MG and the tilting angle of the sub-pump SP to ensure apredetermined discharge amount from the sub-pump SP.

If a signal for lowering the boom cylinder BC is input to the controllerC from the sensor 14 a, the controller C calculates a lowering speed ofthe boom cylinder BC required by the operator according to the operatedamount of the control valve 14, closes the proportional electromagneticvalve 34 and switches the electromagnetic on-off valve 50 to the openposition.

If the proportional electromagnetic valve 34 is closed and theelectromagnetic on-off valve 50 is switched to the open position, allthe return fluid of the boom cylinder BC is supplied to the assist motorAM. However, if the flow rate consumed by the assist motor AM is lowerthan a flow rate necessary to maintain the lowering speed required bythe operator, the boom cylinder BC cannot maintain the lowering speedrequired by the operator. In this case, the controller C controls theopening of the proportional electromagnetic valve 34 to return a flowrate equal to or higher than that consumed by the assist motor AM to thetank T based on the operated amount of the control valve 14, the tiltingangle of the assist motor AM, the rotation speed of the electric motorMG and the like and maintains the lowering speed of the boom cylinder BCrequired by the operator.

If fluid is supplied to the assist motor AM, the assist motor AMrotates. The rotational force of the assist motor AM acts on thecoaxially rotating electric motor MG. The rotational force of the assistmotor AM acts as an assist force for the electric motor MG. Accordingly,power consumption can be reduced by the rotational force of the assistmotor AM.

It is also possible to rotate the sub-pump SP only by the rotationalforce of the assist motor AM without supplying power to the electricmotor MG. In this case, the assist motor AM and the sub-pump SP exhibitthe pressure conversion function.

If the operator suddenly returns the control valves 3, 14 to the neutralpositions to suddenly stop a lowering movement of the boom cylinder BCwith the electromagnetic on-off valve 50 switched to the open position,the electromagnetic on-off valve 50 may not be able to follow, therebycausing a switching delay.

If there is a switching delay in the electromagnetic on-off valve 50,much of the return oil of the boom cylinder BC flows into the connectionpassage 42. Since the pressure of the return oil acts on the regulator36, the displacement volume per rotation of the assist motor AM can bereduced and the torque thereof can be suppressed to or below theabsorption torque of the electric motor MG.

Since the torque of the assist motor AM is suppressed to or below theabsorption torque of the electric motor MG, the pressure of the boomcylinder BC increases to increase the braking action, wherefore there isno likelihood of runaway. A controlling distance can be shortened andthe operation of the operator does not cause a sense of incongruity.

It may take some time until the assist motor AM lowers the displacementvolume thereof. In this case, the pressure in the connection passage 42somewhat increases. However, since the relief valve 52 is so set as tosimultaneously exhibit a relief function, the torque of the assist motorAM does not increase to or above the absorption torque of the generatordue to a switching delay of the electromagnetic on-off valve 50.

Further, since the pressure override of the relief valve 52 issubstantially worsened by the throttle 53, a braking force can beincreased for the boom cylinder BC without any shock.

Next, a case is described where the rotating operation of the rotationmotor RM and the lowering operation of the boom cylinder BC aresimultaneously performed.

In the case of lowering the boom cylinder BC while rotating the rotationmotor RM, the fluid from the rotation motor RM and the return fluid fromthe boom cylinder BC are joined in the connection passage 42 andsupplied to the assist motor AM.

As the pressure in the connection passage 42 increases, the pressure inthe joint passage 43 also increases. Even if this pressure becomeshigher than the rotation pressure of the rotation motor RM or the brakepressure, the rotation motor RM is not affected since the check valves44, 45 are provided.

If the pressure in the connection passage 42 becomes lower than therotation pressure or the brake pressure, the controller C closes theelectromagnetic switching valve 46 in accordance with a pressure signalfrom the pressure sensor 47.

Accordingly, in the case of simultaneously performing the rotatingoperation of the rotation motor RM and the lowering operation of theboom cylinder BC, the tilting angle of the assist motor AM may bedetermined based on a required lowering speed of the boom cylinder BCregardless of the rotation pressure or the brake pressure.

In any case, the output of the sub-pump SP can be assisted by that ofthe assist motor AM and the flow rate discharged from the sub-pump SPcan be proportionally distributed by the first and second proportionalelectromagnetic throttle valves 40, 41 and supplied to the first andsecond circuit systems.

In the case of using the electric motor MG as a generator using theassist motor AM as a drive source, the electric motor MG can exhibit apower generation function utilizing the output of the assist motor AM ifa substantially no-load state is set by zeroing the tilting angle of thesub-pump SP and the assist motor AM maintains an output necessary torotate the electric motor MG.

In this embodiment, it is possible to generate power by the generator 22utilizing the output of the engine E and by the electric motor MGutilizing the assist motor AM. The generated power is stored in thebattery 24. However, since the battery 24 can be charged utilizing thehousehold power supply 25 in this embodiment, the power of the electricmotor MG can be obtained in various ways.

In this embodiment, the assist motor AM can be rotated utilizing thefluid from the rotation motor RM and the boom cylinder BC, and thesub-pump SP and the electric motor MG can be assisted by the output ofthe assist motor AM. Thus, energy loss until regenerative motive poweris utilized is suppressed to a minimum level.

Since the output of the electric motor MG can be controlled according tothe working state from the light working state to the heavy workingstate, the output of the electric motor MG can be relatively reducedparticularly in light work such as ground leveling. Accordingly, batteryconsumption can be reduced and the life of the battery can be extendedby as much as power consumption is reduced.

In some cases, a charge capacity of a battery to be mounted can bereduced to make the battery smaller.

The embodiment of the present invention has been described above. Theabove embodiment is merely illustration of one application example ofthe present invention and not of the nature to specifically limit thetechnical scope of the present invention to the above embodiment.

The present application claims a priority based on Japanese PatentApplication No. 2010-29345 and Japanese Patent Application No.2010-72560 filed with the Japanese Patent Office on Feb. 12, 2010 andMar. 26, 2010, all the contents of which are hereby incorporated byreference.

Industrial Applicability

Application to hybrid construction machines such as power shovels ispossible.

The invention claimed is:
 1. A control system for a hybrid constructionmachine, comprising: a variable-capacity main pump; a circuit systemwhich is connected to the main pump and includes a plurality of controlvalves; a regulator which controls a tilting angle of the main pump; apilot flow path which is provided in the circuit system and introduces apilot pressure generated when any one of the plurality of control valvesis switched to the regulator; an electric motor; a variable-capacitysub-pump which is connected to a discharge side of the main pump anddriven by an output of the electric motor; a regulator which is providedin the sub-pump and controls a tilting angle of the sub-pump; a pressuresensor which is provided in the pilot flow path and detects the pilotpressure; and a controller which is connected to the pressure sensor,controls the regulator of the sub-pump in accordance with a pressuresignal from the pressure sensor, detects an output of the main pump,determines whether a work state is a heavy working state or a lightworking state based on a detected output of the main pump, sets anoutput control value larger when the work state is determined as theheavy working state compared to when the work state is determined as thelight working state, and controls the electric motor based on the setoutput control value.